The invention relates to a charge and discharge control circuit for controlling the charge and discharge of a secondary cell, and to a chargeable power supply device using the charge and discharge control circuit.
A chargeable power supply device has been disclosed, as shown in a circuit block diagram of FIG. 2. In FIG. 2, a secondary cell 201 is connected to an external terminal -V0 or +V0 through a switch circuit 203 and current sensing resistance 204. A switch control circuit 202 and an overcurrent detecting circuit 205 are connected in parallel to the secondary cell 201.
The switch control circuit 202 has a function to detect the voltage of the secondary cell 201, and a signal generates from the switch control circuit 202 so as to turn the switch circuit off at either of an overcharge state and an overdischarge state of the secondary cell 201. A comparator 212 monitors the voltage of the current sensing resistance 204 and compares it with the reference voltage of the reference voltage circuit 206 at overcurrent state caused by an unusual load.
Defining the reference voltage of the reference voltage circuit 206 as Vref [V], the resistance value of the current sensing resistance 204 as R [.OMEGA.] (at this time, ON resistance of the switch circuit 203 should be smaller enough than R), and current flowing there as I [A], when EQU I.gtoreq.Vref/R [A] (1)
output of the comparator 212 turns to "L" from "H" and turns the switch circuit off. Because of that, in an overcurrent state, the switch circuit 203 similarly turns off and stops an energy supply to the load connected to the external terminals -V0 and +V0. Further, in a case that a capacitor 313 is connected to the load as shown in FIG. 3, voltage of the secondary cell 301 falls and becomes overcurrent state because the secondary cell 301 lets current flow in a moment to charge the capacitor 301 if electric charge is not charged to the capacitor 313. To avoid the state, delay time is formed in detecting overcurrent and overcurrent is not detected while electric charge is charged to the capacity. It means that the switch control circuit 302 prevents from overdischarge to the secondary cell 301 and transitional fall of charging ability caused by energy supply to the load connected to the external terminals from the secondary cell 301 by controlling the switch circuit 303 between the secondary cell 301 and the external terminals. When great current is consumed because of unusual load, overcurrent is detected and the switch circuit 303 too turns off to stop discharge.
The prior charge and discharge control circuit has the following defects in detecting overcurrent.
Detecting voltage and delay time of the voltage detecting circuit of overcurrent detecting resistance are set in only one combination depending on current which previously assumed load consumes and time which the current is consumed. There have been a problem that the overcurrent detecting circuit turns the switch circuit off after delay time and the switch circuit breaks because of heat before stopping energy supply to the load when greater load than previously estimated is connected or greater current than previously estimated is consumed.
To solve the above-described problem, an object of the present invention is to provide a charge and discharge control circuit which sets more than two kinds of varying detecting voltage and delay time of overcurrent detecting resistance, detects overcurrent after delay time depending on load current, and stops discharge to stop before the switch circuit breaks because of heat.